Final answer:
Periodic trends refer to the predictable changes in properties such as atomic size, ionization energy, and electron affinity that occur across a row or down a column in the periodic table due to the arrangement of elements by atomic number.
Step-by-step explanation:
The predictable increases or decreases in the properties of elements across a row or down a column in the periodic table are referred to as periodic trends. One of the key aspects of the periodic table is how it showcases these trends due to the arrangement of elements in increasing order of atomic numbers. There are several periodic trends that are of particular interest, such as atomic size, ionization energy, and electron affinity.
For example, atomic size tends to decrease from left to right within a period due to the increasing nuclear charge, which pulls electrons closer to the nucleus. As we move down a group, atomic size generally increases because additional energy levels are added, which are further away from the nucleus. Similarly, ionization energy, which is the energy required to remove an electron from an atom, generally increases across a period from left to right and decreases down a group, as atoms further down have electrons farther from the nucleus and are therefore easier to remove.